Field of the Invention
The invention relates to a telephone which receives audio signals and telephone operating signals via a public telephone network and to a method for processing audio signals and telephone operating signals in a telephone.
The public telephone network is used to transmit various types of telephone operating signals which have completely different signal levels. One of the telephone operating signals is the bell signal, which has an amplitude of 120 V. The DC voltage component of this signal is 60 V. In addition, the public telephone network is used to transmit charge-unit signals (meter pulse signals) whose signal amplitude is 12 V. Furthermore, caller identification signals are transmitted via the network (CID: Caller Identification).
There are historical reasons for the very different signal levels of the individual telephone operating signals. Thus, the bell signal was originally used to actuate mechanical bell devices electromagnetically. The short AC voltage pulses of the meter pulse signal were used as metering pulses for a mechanical charge meter.
On account of the very different voltage levels of the various signals, it is also necessary in modern telephones for these signals to be removed from the reception path using discrete components and processed separately. Hence, to date, every telephone needs to contain separate, discretely designed evaluation circuits for the various types of telephone operating signals.
Added to this, different standards exist for the various telephone operating signals in various countries. First, the signal levels of the bell signal, of the meter pulse signal etc. differ from one another in the various national public telephone networks. Second, the various telephone operating signals are also transmitted in different formats. While some national standards make provision for the caller identifier to be transmitted in FSK (Frequency Shift Keying) format, other standards require that this identifier be transmitted in DTMF (Dual Tone MultiFrequency) format. To be able to evaluate these caller identifiers on the basis of the various national standards, additional, discretely designed evaluation circuits are therefore required. A telephone that is produced in large quantities and needs to be able to be used in various countries therefore needs to be equipped with various discrete evaluation circuits. The advantage of larger quantities is therefore confronted by the drawback of increased component complexity.